Turbo/Increasing Boost

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It is nearly always possible to increase the power of a turbo charged production engine by increasing the boost pressure. The Lotus-Isuzu 4XE1-MT engine in no exception to this. The IHI RHB-5 turbo unit is capable of producing considerably more boost pressure than the 0.65 bar limit set by Lotus. It should be remembered that any increase in the boost pressure will only affect the engine when it’s on boost so the engine’s low speed performance will be unaltered. Many Elan owners who have increased the boost pressure have not experienced any immediate reliability problems in doing so, however there will be more wear particularly on drive-shafts, tyres, brakes and of course the turbo charger. The under-bonnet/hood temperature will also rise! You should certainly not increase the engine’s performance unless the brakes are capable of stopping the car efficiently. It is also important that the engine is tuned and running well before adding performance bolt-ons. Even ECU engines go out of tune, so make sure the plugs, filters, base engine timing, throttle position sensor and CO are correctly set-up before making changes. You will probably discover power you never thought you had!

There are a number of different ways of increasing boost but my advice is: if you decide to increase boost make sure you consider the implications carefully, and only then, choose a boost package from a reputable company.

The standard engine set-up in SE form produces around 165 BHP. One of the most experienced companies in the UK who act as consultants to many manufacturers, state that realistically you can achieve +40 BHP with the Isuzu engine. No doubt more can be achieved but this would probably require internal modifications.

Another factor to consider is 'Torque Steer', which although virtually absent in standard engine trim is more likely to rear its head if power and torque are increased. Using a limited-slip differential is, of course, an expensive counter option. Lotus openly states that 200BHP is a sensible upper limit for Front Wheel Drive.

A point for discussion:

Turbocharged engines are particularly prone to pre-detonation with the increase in intake pressure. Believe it or not, if the pressure becomes too high pre-detonation 'pinging' or 'knock' occurs and output power actually decreases. Pinging is not desirable as it stresses the engine increasing wear. The Elan’s ECU actually checks for pre-detonation and retards the ignition timing and waste-gate modulation frequency to ensure that 'knock' is prevented. There are some grey areas when considering boost increases: The ECU software requires control of the wastegate as a means to prevent engine knock in conjunction with ignition timing. This is clearly shown in the Lotus Service notes. A device called a Boost Control Frequency Valve 'BCFV' vents the wastegate pressure capsule to the atmosphere to enable higher boost pressures before the wastegate activates. In standard set-up this occurs when the ECU allows the boost to increase over the normal spring controlled pressure of 0.41 bar above 2900rpm. If the wastegate capsule is vented directly to the atmosphere without the modulated atmosphere exposure of the BCFV, the boost would quickly rise above 0.9 bar and trigger fuel cut-off; don’t try it! The software in the ECU is therefore theoretically capable of changing the boost curve and maximum boost pressure without any mechanical changes to the turbo system!

Because it's unclear as to how the ECU software is configured, it would appear to me that any bypassing of the ECU's wastegate control, such as a bleed valve, could potentially jeopardise the control system's ability to prevent engine knock. As a practical example, if the base ignition timing is wrong, promoting knock, the ECU will reduce the maximum boost available even in standard tune.

The question that needs to be asked is why so many aftermarket systems rely on physical changes to the wastegate system when revised software should be all that's required ? The Elan's GM sourced ECU is a standard module in which different PROM (Programmable Read Only Memory) ICs can be used, depending on the type of engine being managed. If the system strategy and look-up tables are contained in the PROM, why don't the aftermarket companies offer revised software only ? Are we really being offered the best possible performance increase with the least impact on reliability? What are replacement chips really doing ? If anyone reading this has been involved in developing ECU strategies and mappings, we would be very interested to know the facts.

See Also